1. Field of the Invention
This invention relates to steel sheets for painting such as cold rolled steel sheets, zinc hot dipped or electroplated steel sheets and so on, which are used by the forming such as press forming or the like before or after painting process as an outer panel for automobiles or a decorative outer plate for electric appliances, and a method of producing the same.
2. Related Art Statement
As a typical example of the painting steel sheet, the cold rolled thin steel sheet is usually produced by subjecting the cold rolled steel sheet to degreasing, annealing and temper rolling in this order. In this case, the temper rolling is to improve the galling resistance in the press forming by conducting a light rolling through work rolls having a dulled surface to give a proper surface roughness to the steel sheet surface.
As a process of dulling the surface of the work roll to be used in the temper rolling, there have hitherto been practised a shot blast process and a discharge working process. When the work roll for temper rolling is subjected to a dulling according to these processes, an irregular roughness profile is formed on the surface of the work roll, and consequently the steel sheet after temper rolling indicates a rough surface comprising a plurality of irregular mountain and valley portions as shown in FIGS. 1 or 2. If such a surface roughened steel sheet is subjected to a press forming, a lubricating oil is reserved in the valley portions to reduce friction force between press mold and steel sheet and hence make the press operation easy, while metallic powder separated out by the friction force to the mold is trapped in the valley portions to prevent the galling.
Lately, the finish feeling after painting on vehicle body in passenger cars and trucks is a very important quality control item because the height in synthetic quality of automobile can directly be appealed to the eye of the user as a good finish quality. Now, there are several evaluation items on the painted surface. Among them, it is particularly important that a glossiness lessening irregular reflection on the painted surface and an image clarity defining few image strain are excellent. In general, the combination of the glossiness and the image clarity is called as a distinctness of image.
It is known that the distinctness of image on the painted surface is dependent upon the kind of paint and the painting process but is strongly influenced by the rough surface of the steel sheet as a substrate. That is, when a ratio of flat portion occupied in the steel sheet surface is small and the unevenness is much, the ratio of flat portion occupied in the painted surface becomes small and the unevenness becomes larger, and consequently the irregular reflection of light is caused to damage the glossiness and also the image strain is produced to deteriorate the image clarity, so that the distinctness of image is degraded.
In general, the roughness of the steel sheet surface is frequently represented as a center-line average roughness Ra. Further, it is known that as the center-line average roughness Ra becomes larger, the amplitude between mountain portion and valley portion becomes large and hence the unevenness of the painted surface becomes large and consequently the distinctness of image is degraded.
As a method for evaluating the distinctness of image, there have been developed various systems. Among them, a value measured by means of a Dorigon meter made by Hunter Associates Laboratory or a so-called DOI value is most usually used. The DOI value is expressed by DOI=100x(Rs-R.sub.0.3)/Rs, wherein Rs is an intensity of a specular reflected light when a light entered at an incident angle of 30.degree. is reflected at a specular reflective angle of 30.degree. with respect to a sample S, and R.sub.0.3 is an intensity of a scattered light at a reflective angle of 30.degree..+-.0.3.degree.. The relation between the DOI value indicating the distinctness of image and the center-line average roughness Ra is shown in FIGS. 4 and 5. FIG. 4 is a case that a two-layer coating of 55 .mu.m in thickness is applied to a steel sheet temper rolled with a roll dulled through the conventional shot blast process, and FIG. 5 is a case that a three-layer coating of 85 .mu.m in thickness is applied to the same steel sheet as mentioned above. It can be understood from FIGS. 4 and 5 that as the center-line average roughness Ra becomes large, the DOI value becomes small to degrade the distinctness of image.
When the steel sheet is subjected to a temper rolling with work rolls dulled through the conventional shot blast process or discharge working process, it exhibits a rough surface comprised of irregular mountain portions and valley portions as previously mentioned, wherein the flat portion is very little. When the painting is applied to the steel sheet having such irregular mountain and valley portions, since the coating is formed along the slopes of the mountain and valley portions, the ratio of flat portion occupied in the painted surface becomes small as shown, for example, in FIG. 33 being mentioned later and consequently the distinctness of image is degraded. In the conventional shot blast process or discharge working process, such a problem can not be avoided, so that it is very difficult to provide a sufficiently improved distinctness of image on the painted surface.